This invention relates to methods and apparatus for producing patterns on photosensitive substrates, and relates more particularly to methods and apparatus for generating the different photomasking patterns required for all respective layers of a particular high-density multilayered integrated circuit concurrently under identical environmental conditions to minimize misregistration error.
Mask pattern generating machines currently commerically available employ X-Y stepping of a substrate requiring over thirty hours to complete the exposure of a single mask pattern; and only one pattern can be made at a time. As circuit density increases, this time is expected to increase to over 120 hours per mask, further increasing the likelihood of misregistration errors that can result from changes in ambient conditions occurring from the time the mask for the first layer is started until that for the last layer is completed.
The most pertinent known prior art is U.S. Pat. No. 3,622,742. This patent shows and describes an apparatus in which a modulated laser beam machines, by vaporization, thin films on a plurality of substrates mounted about the periphery of a rotating drum. The beam passes through a lens that is oscillated to change the position of the beam focus to compensate for change in depth of field as the flat-coated surface of the substrate rotates past the beam. The beam is also stepped in an axial direction to impinge successively on different parts of the substrate during successive revolutions of the drum. Code plates are fixed to the periphery of the drum. Each plate has a photosensed timing track and is precisely located by index pins relative to its respective substrate. Clocking pulses for each substrate are thus discontinuous; i.e., they are generated only while a laser beam is passing through slots in a code plate, and hence cannot provide the clock continuity necessary for a high-frequency clocking system required for high-density, fine-resolution circuits.
This cited patent suggests, however, that if desired, the substrates may be mounted on the surface of a disk, rather than the periphery of a drum, and notes that coordinate transformation (such as employed in the present invention) would then be necessary to produce rectilinear patterns. The patent further states that: "Normally all of the circuits to be machined are identical, and if this be the case, the modulation of the beam is identical through each successive substrate that intercepts the beam during one drum rotation." Even if this is construed to imply that the substrates may differ, there clearly is no teaching or even remote suggestion that the substrates would be so related as to fabricate in the same exposing operation all of the photomasks necessary for making a particular multilayered circuit. Moreover, no prior art is known which suggests making all such related masks concurrently in the same exposing operation. Nor is there any teaching of a continuous clock arrangement to insure, with a permissible limited degree of self-regulation, precision registration of patterns on the respective masks.